Overview
I'm assuming you know your Linux commands.
This system will not boot off the RAID6, in fact we're assuming that some how /dev/sda and /dev/sdb are in some bootable RAID1 type setup.
In this example we'll use the 15 other drives in our system to make a very large RAID6.
Creating RAID6 with mdadm
This example has 15 physical disks, /dev/sdc through /dev/sdq. When making a RAID6, we'll use the entire disk, no partitions.
mdadm \ --create /dev/md1 \ --level=6 \ --raid-devices=15 \ /dev/sdc /dev/sdd /dev/sde /dev/sdf /dev/sdg /dev/sdh /dev/sdi /dev/sdj /dev/sdk /dev/sdl /dev/sdm /dev/sdn /dev/sdo /dev/sdp /dev/sdq
Personalities : [raid6] [raid5] [raid4]
md1 : active raid6 sdq[14] sdp[13] sdo[12] sdn[11] sdm[10] sdl[9] sdk[8] sdj[7] sdi[6] sdh[5] sdg[4] sdf[3] sde[2] sdd[1] sdc[0]
50789533184 blocks super 1.2 level 6, 512k chunk, algorithm 2 [15/15] [UUUUUUUUUUUUUUU]
[>....................] resync = 0.0% (878004/3906887168) finish=5635.6min speed=11551K/sec
unused devices:
Now we just wait for this thing to finish rebuilding.
mkfs.xfs --